Micro-scale lenses include, for example, lenses having a diameter ranging from about 4 millimeters (mm) down to about 20 micrometers or microns (μm). Microlenses have many valuable applications, which include but are not limited to, monitors for light beam distortion or wavefront aberrations for ophthalmic analysis systems and for adaptive optics systems for astronomy. Specifically, one type of useful application of a regularly spaced array of microlens is a Shack-Hartmann (SH) sensor.
Shack-Hartmann sensors are widely used to measure, and even to continuously monitor and correct, wavefront aberrations. Adaptive optical systems perform real-time corrections on wavefront aberrations. SH sensors are fast, accurate, and (in contrast to interferometers) generally insensitive to vibrations. When SH sensors are used in conjunction with adjustable optical devices, such as an array of small mirrors each of which can be adjusted, the image quality of a telescope can be very substantially improved. An adaptive astronomical telescope performs real time corrections on the wavefront aberrations which are always introduced as a beam of starlight traverses the earth's atmosphere.
Shack-Hartmann sensors have proven to be the most suitable wavefront monitors for ophthalmic analysis applications, such as used for keratoconus analysis (a disorder including having an eye with a cone shaped cornea), and for both before and after laser-assisted in-situ keratomileusis (LASIK) surgery. Fast measurement speed, relatively low light illumination levels, and extremely high accuracy are required when measuring the optical aberrations as a light beam passes through human eyes. Despite asking the patent to hold still, eyes are constantly moving at the scale of distance that is important in ophthalmic analysis.
FIGS. 1A and 1B show a conventional Shack-Hartmann sensor according to the background art. These figures also show how a light beam with high aberration is erroneously analyzed by such a sensor. FIG. 1A is a cut away side view, where the cut is made along cut line 1A as shown in FIG. 1B.
SH sensor 100 includes a microlens array 120 comprising a handling layer 125 and a 3-by-3 array of microlenses 126. Microlens array 120 dissects incoming light beam 110 into a number of segments 114 and focuses each segment onto a spot or focal point 116. That is, each light bean segment 114 is diffracted by a corresponding lens 126 to produce a focal spot or image 116 on opto-electronic detector 140.
Detector 140 can be, for example, an array of charge-coupled devices (CCDs), or of complementary metal oxide silicon (CMOS) detectors. The detector area 145 that is assigned to each microlens is sometimes referred to as the microlens' sub-aperture. Since the microlens array and the detector array are typically both closely packed rectangular (or even more closely packed hexagonal arrays), each assigned area is comparable to the area of one of the microlenses. For example, the assigned area within a CCD may include around 40 CCD pixels formed by, for example, a 6 by 6 or a 7 by 7 array of pixels.
In FIG. 1A, the curvature or aberration of each segment 115 is represented by an arrow 115. Each arrow 115 is normal to the average direction of the wavefront of that particular light beam segment 114 at the plane where that particular segment is incident on array 120. Thus, the pattern of focal points 116 contains information about the waveform aberration.
This waveform aberration information is spatially resolved, and can be analyzed to reconstruct the wavefront over the area of micro lens array 120. If there were zero optical distortion in the path that light beam 110 takes from its source to array 120, then each arrow 115 would be horizontal and normal to the surface of array 120 and each focal point 116 would be at the exact center of its assigned area 145 within opto-electronic detector 140. In normal operation, the focal point of a microlens is not at the center of its assigned area, but it does fall somewhere within its assigned area. However beam 110, as shown in the example in FIG. 1, has high aberration.
The wavefront aberration of light beam 110 includes light beam segment 114A entering array 120 with a very large downward wavefront curvature, that is, downward with respect to the planes of FIGS. 1A and 1B. This is represented both by a large downward slope of arrow 115A and by the large downward displacement of focal point 116A. Segment 116A also has a slight rightward curvature represented by slightly off to the right position of focal point 116A, that is, rightward with respect to the plane of FIG. 1B. Similarly segment 116B has a moderate down and right curvature, represented by the moderate downward slope of arrow 115B and moderate down and right displacement of point 116B.
High aberration of a light beam can be produced by, for example: strong atmospheric disturbance of a light beam from a an astronomical source; defects in an optical device through which the light beam passes in an optics quality assurance or measurement system; or by optical defects in an eye (which is after all another form of an optical device) that is being evaluated in an ophthalmic-analysis system.
A problem arises, as shown in FIGS. 1A and 1B, when the wavefront aberration of the light beam incident on a microlens is large enough that the focal point falls outside of the microlens' assigned area and falls into the assigned area of a different, typically adjacent microlens. In such a situation, a false reading is likely to occur because the detector has no way to associate a focal-point reading with any microlens other than the microlens that is predetermined to be associated with the microlens' assigned area.
In today's world, it is highly desirable to expand the dynamic range of Shack-Hartmann sensors and to thus increasing the maximum amount of wavefront distortion that they can measure accurately. This is especially important in consideration of the ever increasing use of SH sensors in refractive surgery and in the research, diagnosis, and treatment of ophthalmic diseases. In the case of LASIK surgery, the development of a transition zone, formed by scar tissue, at the boundary separating surgically treated and untreated areas often results in very large optical aberrations over limited spatial regions. Ophthalmic conditions, such as keratoconus, require large dynamic ranges and sensitivities that cannot be fully achieved by conventional Shack-Hartmann sensors.
Another problem with respect to microlens arrays is to provide maximum fill-factor and clear aperture. A greater area of coverage of microlenses over the total area of the array, allows more efficient, accurate and sensitive measurements to be made.
Yet another problem is the precise, efficient, economical, and reliable fabrication of wavefront sensors, of the microlens arrays that are used in such sensors, and of the individual microlenses themselves.